Polarizers are important optical components used in polarizing optics to produce the state of linear polarization.
Beamsplitters for amplitude division of an incident light beam, based on the use of birefringent materials, are known in the art. In general, two principals structural shapes are used.
The beamsplitter prisms according to Wollaston, Sernamont or Rochon separate a light beam after its entry into the crystal under a fixed, constant split angle. The Wollaston prism consists of two calcite prisms which are cemented together on their base. Their crystallographic-optical axes lie perpendicular to each other and perpendicular to the direction of propagation of the incident light. Light striking the surface of incidence at right angles is refracted in the first prism into an ordinary (o) and an extraordinary (eo) beam. However, these two beams continue to propagate in the same direction. As the optical axis of the second prism is perpendicular to that of the first, the ordinary beam becomes an extraordinary beam at the boundary of the surface. Its refractive index changes from n(o) to n(eo). The opposite applies to the original extraordinary beam, now an ordinary beam; overall, therefore, the two partial beams are refracted onto different directions. A polarization interferometer using a Wollaston prism is, e.g., disclosed in U.S. Pat. No. 4,732,481 to Matsui et al.
When using a so-called Beam Displacing Prism, e.g., a plan-parallely grounded calcite crystal, the incident light beam is separated by the prism under an angle as well; however, the two partial beams are parallel to each other when leaving the crystal.
In both cases, the separation of the incident light beam into two polarized partial beams of perpendicular polarization is fixed due to the geometry and the characteristics of the material and cannot be varied continuously.
Various optical inspection tools used for production and quality control employ such beamsplitters. One example of such a tool is the so-called MIPS (Miniature Interferometric Phase Sensor—U.S. Pat. No. 5,392,116 to Makosch) used e.g., for the quality control of hard disk read/write heads. The characteristics of the slider flying over the rotating hard disk are determined by its aerodynamically shaped underside, the so-called ABS (air bearing surface). The structures of the ABS are set up by various techniques of surface treatment.
The measurement principle of the MIPS is based on phase-shifting laser interferometry. The beam of a laser diode is split by a Wollaston prism into two partial beams which are perpendicularly polarized to each other. An optical system consisting of various optical elements is employed to focus the two laser beams perpendicularly on the object surface. The two laser spots are moved across the object surface by an internal scanner unit, so the surface profile can be measured. The MIPS system is used above all for detection of edges an different ABS pads. The edge's distances are evaluated and have to be compared with pre-set reference values. Deviations from these reference values will lead to a selection of defective sliders. These inspections guarantee the quality of the outgoing read/write heads.
However, when using the MIPS system for other problems, e.g., the interferometric measurement of the etch depth of the ion mill edge of a slider, the distance between the measurement beam and the reference beam has to be adapted. When using a 1° Wollaston prism (i.e., a prism having a separation angle of 1°), due to the optics used, a beam distance on the object surface of about 25 μm will result. However, in order to determine the etch depth, the resulting measurement signal will have to be reworked, i.e, the differential signal will have to undergo a special evaluation procedure, whereby the excellent reproducibility of the MIPS system is at least partially lost.
The necessity of the mentioned special evaluation algorithm and the faults associated therewith are due to the fixed distance of the two laser beams on the surface of the object to be investigated. Referring to the measurement of the etch depth of the ion mill edge of a slider, this means that the second beam (reference beam) already hits the edge before the first beam has left it.
This problem could be overcome by the use of a 2° Wollaston prism leading to a beam distance of about 50 μm. However, due to other measurement problems it will be more useful to maintain the beam distance of 25 μm. Since a quick change between different Wollaston prisms is impossible due to the high requirements with respect to adjustment of the system this solution is more or less impracticable. It also has to be mentioned that this problem is not only associated with the MIPS system, but applies to other measurement systems using such prisms as well.
Also known in the art is the Soleil-Babinet Compensator (SBC). The SBC is a variable, zero-order waveplate which operates in a similar fashion as a conventional net zero-order waveplate set. The SBC uses birefringent materials such as quartz, MgF2, CdS, or CdSe to produce relative retardation between the two transmitted linear polarization components (extraordinary and ordinary beams). The crystallographic-optical axis of the material thereby forms an angle of 0° or 90° with the plan-parallel surfaces of the crystal. The SBC can be used to introduce a desired ellipticity to a beam of polarized light, analyze the polarization state of light, evaluate fixed retardation plates, measure birefringence in optical windows and crystals and the like.
The internal optics consist of a pair of long and short wedges, which are fabricated with identical angles and are subsequently aligned parallel to each other within the SBC. A parallel plate is then added which has its optic axis orthogonal to that of the wedged pair. Retardation of the system is varied by translating the long wedge perpendicular to the beam direction which effectively increases or decreases the net thickness of the wedged pair depending on the direction of the travel. The thickness of the parallel plate is chosen to match the net thickness of the wedged pair near the thin end of the long wedge. Any translation of the long wedge from this net zero retardation position then introduces a net imbalance in the optical pathlengths of the ordinary and extraordinary beams which creates the finite (zero order) retardation.
However, a spatial separation of the two beams is not possible with the SBC and is even unwanted. The crystallographic-optic axes of the two wedges are arranged perpendicular to the direction of the incident light beam to avoid a spatial separation of the ordinary and the extraordinary beam. Thus, the SBC is not a beamsplitting optical element but a variable phase retardation plate.
Accordingly, the prior art does not provide for a variable lateral separation of the produced ordinary and extraordinary beam.